Downhole seismic sources are used to determine underground geological characteristics in the region surrounding the well in which the source is placed. They are designed to create acoustic waves and to transmit these waves as fully as possible to the inside wall of the well. Receivers such as accelerometers, geophones, or hydrophones detect these acoustic waves after they have passed through the subsoil. The measured waves serve to determine the characteristics of the underground formations passed through.
Conventional acoustic sources generate seismic waves from the surface. These surface seismic sources operate at low frequency resulting in low resolution surveys. A few downhole seismic sources have been developed which generate acoustic waves into the formation through a borehole medium. These downhole sources operate at a higher frequency than surface sources but do not generate enough energy to result in accurate surveys. Accordingly, these downhole sources tend to suffer from poor signal to noise ratios. Further, much of the acoustic energy remains in the well as tube waves and does not penetrate the earth formation.
Conventional downhole sources include:                the cylindrical bender source using piezoelectric ring bonded on a tube developed by Southeast Research Institute and described in Balogh et al.'s, “New Piezoelectric Transducer for Hole-to-Hole Seismic Applications,” 58th Annual International Meeting of the Society of Exploration Geophysics (1988), Session DEV2.5 (incorporated by reference herein in its entirety);        the electroacoustic transducer cylindrical bender source developed by Kompanek and described in U.S. Pat. No. 4,651,044 (incorporated by reference herein in its entirety);        the arc discharge pulse source developed by Southwestern Research Institute as described in U.S. Pat. No. 5,228,011 (incorporated by reference herein in its entirety);        the swept frequency borehole source developed by Western Atlas and described in Owen et al.'s, “Arc Discharge Pulse Source for Borehole Seismic Applications,” 58th Annual International Meeting of the Society of Exploration Geophysics (1988), Session DEV2.4 (incorporated by reference herein in its entirety);        the potential energy “drop mass” source developed by Institut Francais du Petrole (IFP) as described in U.S. Pat. No. 4,505,362 (incorporated by reference herein in its entirety);        the hammer launched source developed by OYO Corporation and described in Kennedy et al.'s “A Swept-Frequency Borehole Source for Inverse VSP and Cross-Borehole Surveying,” 7th Geophysical Conference of the Australian Society of Exploration Geophysics (1989), Volume 20, pages 133-136 (incorporated by reference herein in its entirety); and        the orbital vibrator developed by Conoco and described in Ziolkowksi et al.'s “Determination of Tube-Wave to Body-Wave Ratio for Conoco Borehole Orbital Source,” 69th Annual International Meeting of the Society of Exploration Geophysics (1999), pages 156-159 (incorporated by reference herein in its entirety).        
In general these sources are high energy sources operating at low frequency and are excited using a pulsed technique. Most of these sources are used for wireline applications.
A few sources have been developed that are clamped up against the borehole wall. Because these sources are generally more difficult to design, few sources have been developed. One source utilizes a hydraulic vibrator clamped against the borehole to oscillate a reaction mass axially or radially and is more fully described in Turpening et al.'s “Imaging with Reverse Vertical Seismic Profiles Using a Downhole, Hydraulic, Axial Vibrator,” 62nd Meeting of EAGE (2000), Session P0161 (incorporated by reference herein in its entirety).
Most of the sources listed above behave like monopole sources, with the exception of the orbital vibration and the clamped vibrator sources which behave like dipole sources.
Accordingly, it is one object of the present invention to provide a downhole transducer source capable of providing the resolution required to adequately characterize the earth formation.